Creep resistant steel



CREEP RESISTANT STEEL John R. Rait and John 0. Ward, Sheflield, England, as-

signors to Hadfields Limited, Sheilield, England No Drawing. Application August 20, 1953,

Serial No. 375,562

Claims priority, application Great Britain August 22, 1952 .4 Claims. (Cl. 75-128) nited States Patent ent when a smaller proportion of chromium is used and,

in fact, leads to the formation of two chromium types of carbides, one having a trigonal structure and the other a complex cubic structure. The use of other elements forming face-centred cubic carbides has, therefore, been This invention relates to ferritic alloy steels having tried h, for p e12% hr l y nimproved resistance to scaling and creep deformation at high temperatures.

By a ferritic alloy we mean one which consists of an iron-rich matrix with excess carbide or other compounds taining niobium shown at C in the table. A substantially increased creep resistance has thus been obtained but one which still gives scope for much improvement.

It is well known that thedesirable treatment of such as dispersed particles. Compounds, other than carbides, alloys Consists of a high temperature 801115011 treatment which may be present include nitrides, borides, and intermetallic compounds such, for example, as FeCr commonly referred to as sigma phase in ferrous alloys having high chromium contents. The matrix is usually in followed by a tempering or ageing treatment. For example, the solution treatment may be carried out above 1150 C. provided the temperature employed avoids overheating, burning, or excessive grain coarsening. It is the body-centred cubic condition but may be more-or also well known that the addition Of 12% chromium 10 less distorted by heat treatment; for example, after a. very rapid cool, it may take the tetragonal form known as martensite. Such an alloy is magnetic at room temperature and will, on heating, transform into the noniron decreases the temperature and composition ranges at which austenite is stable. Since the creep resistance prop erties of these alloys depend on the different solubilities of carbon in ferrite and austenite, an alloy which does magnetic face-centred cubic condition known as austenite not become Wholly austenitic at the Solution frfiatmcnt and revert to the magnetic form on cooling. Some ferritic steels are hardenable by heat-treatment; others are not. When hardened, they may assume a variety of forms known as martensite, bainite, troostite, sorbite, pearlite,

temperature but develops a duplex ferritic-austenitic structure, cannot develop its optimum properties. The alloy shown at C in the table is prone to form this duplex structure on heating; to the 'desired temperatures for and so on. One ferritic alloy which has been propos d carbide solution. The ferritic tendency may be avoided for service at high temperatures up to about 700C. has a chromium content of 3% and consists principally of carbide formers and ferrite strengtheners, no attention being given to scale resistance. An example of it is shown at A in the following table:

Table by the addition of nitrogen or nickel or other austenitising elements and a very substantial increase in creep resistance be thereby obtained as can be seen from the alloy shown at D in the table.

40 Whereas the cubic carbide has formerly been provided The alloy shown at A and, generally speaking, .all

Total Creep Deformation, per- Comparative cent at times Creep Re- Analysls stated under a sistance tensile stress of based on8 8 tons/sq. in. at tons/sq. in. 600 C.

and 300 hrs.

0 Cr Mo W V Nb Ti N B 300 hrs 1000 hrs by two carbide formers, i. e. niobium and vanadium, we

similar ferritic alloys with good creep resistance have finclthat a mixture ofthree or more face-centred cubic poor scaling resistance.

It has been clearly shown in published work thatif such an alloy is to have a good creep resistance at high temperatures, its composition must be adjusted so that carbides is preferable. The carbide formers can, for ex ample, be titanium, vanadium, and niobium. The use of titanium enables a reduced amount of niobium and vanadium to be employed with a further substantial inthe optimum amount of carbide of a face-centred cubic crease increep resistance-see the alloy shown at E in structure is formed, i. e. so that the carbides present have a structure similar to that of sodium chloride. The creep resistance is dependent upon the combination of the constitution of the matrix, the crystal form, size disthe table. A still further increase in creep resistance is obtained by the addition of boron as shown at F in the table. i

The use of titanium as a carbide former is of the greattribution and composition of the carbides, these factors estimportance both technically and commercially. Techbeing dependent upon the manner in which the alloy is heat treated.

nically, as canbe seenffrom the table, it leads to a very substantial increase, in creep resistance. Commercially,

Patented May 21, 1957 its importance is that it is both cheap and abundant in comparison with niobium and tantalum and, as its use allows the amounts of the latter elements to be substantially reduced, it has a substantial bearing on the cost of production of the alloys.

The addition of the titanium is preferably made as titanium metal to the molten steel after deoxidation of the bath by means of calcium-silicon-Zirconium or other suitable deoxidant. Ferro-titanium may be used but the titanium metal is preferred since ferro-titanium commonly contains impurities which may have a deleterious effect on the creep resistance.

The table given above gives in Examples E and F the analysis of particular alloys in accordance with the invention and is concerned only with essential elements. In general, the invention extends to ferritic steel alloys having 'high creep resistance and scaling resistance at temperatures up to about 700 C. and having as essential constituents:

Percent Carbon 0.05-0.50 Chromium 8.0-17.0 Molybdenum 4.0

or Tungsten 2.0

Molybdenum Tungsten Nitrogen 0.01-0.25 Silicon 0.1- 2.0 Manganese 0.1- 4.0

Three or more carbide formers including titanium and selected from vanadium, niobium, tantalum, titanium, hafnium, zirconium 0.05-4.0

The alloys may also usefully contain:

And unavoidable impurities.

The heat treatment of such an alloy-should comprise a high temperature treatment followed by a tempering or ageing treatment.

The alloys in accordance with the invention can be used under high stress and high temperature conditions such as occur in gas turbines and other applications.

The following is a typical example of results obtained from the invention:

A disc approximately .24 inches in diameter and 2 inches thick with an integral boss 8 inches in diameter and 4 /2 inches thick Was forged from an ingot inches square of a steel manufactured by the basic electric arc process having the following percentage composition:

The disc was quenched from 1250" C. and tempered for 48 hours at 650 C. It was then found to have the following properties:

Tangen- Transtial Radial verse at centre Creep Deformation (percent) under a tensile stress 8 tons/sq. in. at 300 hours at 600 C 0.073 0 078 0.081 Tensile Strength at room temperature (tons/sq. in):

0.1% proof stress 52. 4 51. 6 49. 9 max. stress 62. 9 62.2 60. 0 Elongation (percent) (L=4 /Z) 15. 5 16.0 15.0 Reduction of Area (percent) 42. 9 37. 9 41. 3

We claim:

1. A ferritic alloy steel having high scale resistance and creep resistance up to about 700 C. and having as essential constituents:

Percent Carbon 0.050.S0 Chromium 8.0 17.0 Nitrogen 0.01-0.25 Silicon 0.1 2.0 Manganese 0.1 4.0

a substance selected from the group consisting of molybdenum (not greater than 4%), tungsten (not greater than 2%) and molybdenum tungsten (not greater than 4%), from 0.05 to 4% of at least three face centered cubic carbide formers including titanium and selected from the group consisting of titanium, vanadium, niobium, tantalum, hafnium, and zirconium, and the remainder being essentially iron.

2. A ferritic alloy steel according to claim 1 containing also any one or more of the following:

Percent Nickel 2.0 Boron O.5 Cobalt Copper 20 3. A ferritic steel alloy having the percentage composition:

And unavoidable impurities.

4. A ferritic alloy steel containing 0.05-0.50% carbon, 8.0-17.0% chromium, a substance selected from the group consisting of molybdenum (not greater than 4%),

5 tungsten (not greater than 2%) and molybdenum tungsten (not greater than 4%), from 0.05 to 4% of at least three face centered cubic carbide formers including titanium and selected from the group consisting of titanium, vanadium, niobium, tantalum, hafnium, and zirconium, the balance substantially iron.

References Cited in the file of this patent UNITED STATES PATENTS Urban Feb. 20, 1951 Krainer Nov. 27, 1951 Kirkby et a1. Apr. 1, 1952 

